asset support for unpacking a single docker layer

[dexhorthy]

What I Did

• add support for unpacking a single docker layer to a target destination

---
assets:
  v1:
    - dockerlayer:
        dest: deb/docker/
        image: quay.io/replicated/docker-packages:ubuntu-1604-v1.12.3.20180417
        source: public
        layer: f7126e84abc96fbc8495c33052724fad48115829e86987adbf556474f0ead5c1

lifecycle:
  v1:
  - render: {}

How I Did it

• add new asset DockerLayer, which extends DockerAsset
• move some docker helper structs and interfaces into a new images package
• Refactor planner.dockerStep into docker package as docker.Renderer so we can reuse it for dockerlayer
• add dockerlayer.Unpacker struct that can unpack a docker layer, delegating to the docker.Renderer

The Unpacker does the following:

• docker save the image to a tmp directory
• untar the image.tar into a second tmp directory
• untar the second tmp directory/<layer sha>/layer.tar into the asset's dest

Other stuff

• we were missing a fail with an error on unsupported plan steps, had to refactor the planner.Build() to be able to return an error
• some other random refactors and improvements

How to verify it

• Run the example in examples/dockerlayer/packages.yml
• write more assets that use dockerlayer

Description for the Changelog

Added support for dockerlayer assets, which

Picture of a Boat (not required but encouraged)

[codeclimate]

Function TestDeserialize has 122 lines of code (exceeds 50 allowed). Consider refactoring.

[dexhorthy]

/wontfix

[codeclimate]

Method Unpacker.save has 5 arguments (exceeds 4 allowed). Consider refactoring.

[codeclimate]

Function NewUnpacker has 5 arguments (exceeds 4 allowed). Consider refactoring.

[codeclimate]

Function TestUnpackLayer has 70 lines of code (exceeds 50 allowed). Consider refactoring.

[codeclimate]

Function NewStep has 5 arguments (exceeds 4 allowed). Consider refactoring.

[codeclimate]

Code Climate has analyzed commit 1eea0d4 and detected 11 issues on this pull request.

Here's the issue category breakdown:

Category	Count
Complexity	9
Duplication	2

The test coverage on the diff in this pull request is 67.9% (50% is the threshold).

This pull request will bring the total coverage in the repository to 48.5% (-0.2% change).

View more on Code Climate.

[dexhorthy]

I reviewed CC and they're all "this table test is too long"